This application relates to semiconductor quantum well devices for converting images from one spectral region into images in another spectral region.
Much work has been done recently on a wide range of electro-optic devices based on the electric-field dependence of strong absorption resonances in semiconductor quantum wells (QWs). In a QW, a thin layer of one semiconductor material is sandwiched between cladding layers of a different material, with the electronic properties of the materials being such that an electric potential well (in the central layer) is formed between two electric potential barriers (in the cladding layers). The Qw's small thickness, on the order of 100 .ANG., results in quantization of charge-carrier motion in the thickness direction.
Also, QWs exhibit the quantum-confined Stark effect, in which the wavelengths of the QW's peak optical absorptions associated with the creation of light- and heavy-hole excitons shift to longer wavelengths in response to an applied electric field. Since these peak excitonic absorptions have finite spectral widths due to electron/hole interactions with material impurities and phonons, the transmissivity of a QW at a wavelength near a peak varies as the applied field varies. These and other aspects of QW devices are described in commonly assigned U.S. Pat. No. 5,047,822 to Little, Jr., et at., which is expressly incorporated here by reference.
Because a single QW is so thin, devices are typically made by stacking a number of Qws, e.g., fifty, to obtain significant optical effects. Many aspects of multiple quantum well (MQW) devices are described in the literature, including C. Weisbuch et al., Quantum Semiconductor Structures, Academic Press, Inc., San Diego, Calif. (1991).
A simple MQW device is the absorption modulator, in which the excitonic absorption edge of the quantum wells is moved into and out of coincidence with the wavelength of a spectrally narrow light source, such as a laser, by varying an applied electric field. Thus, the intensity of the light transmitted or reflected by the modulator varies according to the applied electric field, or bias voltage, as noted above.
One such absorption modulator, although based on Wannier-Stark localization rather than the quantum-confined Stark effect, is described in K.-K. Law et at., "Normally-Off High-Contrast Asymmetric Fabry-Perot Reflection Modulator Using Wannier-Stark Localization in a Superlattice", Applied Physics Letters vol. 56, pp. 1886-1888 (May 7, 1990); and K.-K. Law et al., "Self-Electro-Optic Device Based on a Superlattice Asymmetric Fabry-Perot Modulator with an On/Off Ratio&gt;100:1 ", Applied Physics Letters vol. 57, pp. 1345-1347 (Sep. 24, 1990). In contrast to the QW's shift of the excitonic absorption peaks to longer wavelengths due to the quantum-confined Stark effect, Wannier-Stark localization leads to a shift to shorter wavelengths for increased electric field in superlattice structures.
In general, a superlattice is a stack of interleaved thin barrier layers and QWs in which the QWs are resonantly coupled, causing the Qws' discrete charge-carder energy levels to broaden into minibands. Applying an electric field destroys the resonance, misaligning the energy levels in neighboring Qws and localizing them over a few Qws. This changes the optical absorption spectrum from a smooth, miniband profile to a peaked, QW-excitonic profile and blue-shifts the absorption edge.
As described in more detail below, Applicants' invention can be embodied using either MQW or superlattice structures. Also, it will be understood that such structures described in this application can be fabricated by a wide variety of semiconductor processing methods, e.g., metal-organic chemical vapor deposition, molecular beam epitaxy, and electrochemical deposition methods. See, e.g., J. Switzer et al., "Electrodeposited Ceramic Superlattices", Sci. vol. 247, pp. 444-445 (Jan. 26, 1990); and the abovecited Weisbuch et al. book.
Simple MQW absorption modulators operating at room temperature can exhibit modulation depths, i.e., ratios of minimal to maximal absorptions, of about 10:1 to 30: 1. These low modulation depths can be improved by combining an MQW structure with a suitable resonant optical cavity, such as an asymmetric Fabry-Perot etalon (ASFPE). An ASFPE is a resonant optical cavity formed by two planar mirrors that have different reflectivities. Such devices are described in commonly assigned U.S. patent application Ser. No. 08/109,550 filed Aug. 20, 1993, by Terranee L. Worchesky and Kenneth J. Ritter, which is expressly incorporated here by reference.
Another application of QWs is the quantum well infrared photodetector (QWIP). In the QWIP described in the literature, including Semiconductor Quantum Wells and Superlattices for Long-Wavelength Infrared Detectors, M. O. Manasreh, ed., pp. 55-108, Artech House, Boston, Mass. (1993), internal photoemission of electrons from bound states in GaAs QWs into high-mobility channels in the QWIP's cladding layers increases the conductivity of the QWIP in the presence of thermal light, i.e., long-wave infrared (LWIR) wavelengths from about 8000 nm to about 12000 nm or mid-wave infrared (MWIR) wavelengths from about 3000 nm to about 5000 nm. The light is detected as an increase in the current flowing through the QWIP when operated at a fixed bias voltage. The characteristics of the QWIP (e.g., the peak-response wavelength, the optical bandwidth, and the electrical properties) are determined by the widths of the QWs (usually in the 4- to 8-nm width range) and the composition of the cladding layers (nominally thick layers of Al.sub.(x) Ga.sub.(1-x) As, with x ranging from 0.2 to 0.6).
Like a charge-coupled device (CCD) imager, arrays of QWIPs can be provided to form thermal images. In a conventional thermal imaging system, an array of detector elements is mated to a silicon multiplexer that reads out the current from each element sequentially in a "bucket brigade" fashion (i.e., the charge is collected from each element into a capacitor and then passed along a row of capacitors to a column-readout capacitor bank which passes it down to a single charge-measuring element on the multiplexer). The original position of each charge packet is tracked, and the image is reconstructed electronically, usually as a video image on a monitor.
Multiplexers optimized for the electrical characteristics of QWIPs are not currently available. The multiplexers that have been used are not well suited for the relatively high dark current typical of QWIPs operating at temperatures around that of liquid nitrogen, and greatly increase the cost of the imaging system. Further, because the multiplexers are made from silicon instead of GaAs, the thermal-expansion-coefficient mismatch limits the arrays' physical sizes to well below the limit imposed by GaAs crystal-growth and processing technology. In addition, the multiplexers must also be cooled since they must be located as close to the detectors as possible, but the multiplexers' thermal mass and heat dissipation strain conventional cooling systems.
Applicants have recognized that the current flowing through a QWIP can be used to provide the bias necessary for an MQW modulator. In such a device, the change in QWIP current due to a change in the amount of MWIR or LWIR light illuminating the QWIP will change the amount or phase of near-infrared (NIR) light, i.e., wavelengths from about 800 nm to about 2000 nm, reflected or transmitted from the MQW modulator. The change in intensity of LWIR or MWIR light is thus converted into a change in intensity or phase of NIR light.
The publication, V. Gorfinkle et at., "Rapid Modulation of Interband Optical Properties of Quantum Wells by Intersubband Absorption", Applied Physics Letters vol. 60, pp. 3141-3143 (Jun. 22, 1992), describes the theory of a doped MQW absorption modulator in which the interband absorption strength for NIR photons would be modulated by intersubband absorption of LWIR photons. The LWIR absorption would partially deplete the population of carriers in the ground state, thereby changing the density of final states for NIR absorption.
A significant drawback of such a device for a purpose such as converting LWIR information into NIR information would be the interdependence of the operating LWIR and NIR wavelengths due to the absorptions occurring in the same MQW structure. Moreover, a very large LWIR flux and fabrication in a waveguide geometry are needed for significant NIR absorption modulation.